Ideal Gases v. Real Gases

■ The good news is that we will deal with ideal gases in Pre-AP chemistry – makes our life and calculations much, much simpler. !

■ The bad news is that you still have to know the differences between ideal and real gases. !

■ See next slide for those differences.

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Ideal Gas v. Real Gas

■ Ideal Gases ■ 1. Size of particles is

negligible. ■ 2. No attraction or

repulsion of particles. ■ 3. Particles move

independently in straight line and just bounce randomly.

■ Real Gases ■ 1. Size matters –

individual particles have volume.

■ 2. There is attraction between particles.

■ 3. Not necessarily random and independent motion.

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Gas Laws

Gas Pressure in General Dalton’s Law Charles’s Law Boyle’s Law

Gay-Lussac’s Law Combined Gas Laws

Graham’s Law Ideal Gas Law

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Gas Pressure in General

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Gases Exert Pressure Equally In All Directions

■ We have seen that gas particles move rapidly and randomly and take the shape of their container.

■ Gas molecules produce a force as they strike the walls of a container. The random motion of gas molecules exert force on the walls of the container.

Let’s take a minute to define pressure.

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Fluids Exert Pressure

■ As the gas particles strike the surface of the container, they exert a force over the surface area of the container, evenly in all directions.

■ Pressure – the amount of force exerted on a given area of a surface.

■ We measure the force exerted by fluids like gases and liquids as pressure.

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Units of Pressure

Pressure – the amount of force exerted on a given area of a surface.

P = F A

Force is measured in Newtons in the SI system.

Area is measured in: •Square meters (m2) or centimeters (cm2) in SI •Square feet (ft2) or inches (in2) in English

In the English system force is commonly measured in pounds.

Named, of course, for Sir Issac Newton.

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SI Units of Pressure

P = F A

Therefore the units of pressure in the SI system are:

N cm2

N m2or

N m2 has a special name in the SI system

1N m2

= 1 Pascal (Pa) Named for Blaise Pascal, a French mathematician.

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SI Unit of Pressure – The Pascal

Since 1N is a pretty small force and 1 m2 is a pretty large area, 1 Pascal is a pretty small unit of pressure.

1 Pa is about the same amount of pressure as the weight of 40 pennies on the top of your class room table.

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KiloPascals

Since the Pa is such a small unit of pressure, often the kiloPascal (kPa) is used instead

1 kPa = 1000 Pa

A crate has a weight of 3000 N (about 750 lb.) and covers an area of 2 m2.

What is the pressure under the crate?

P = F A

= 3000 N 2m2

= 1500 N m2

= 1500 Pa = 1.5 kPa

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Pressure in English Units

P = F A

The units of pressure in the English system are:

lb in2

lb ft2or

Pounds per square inch is often abbreviated as “psi”

1 psi = 6.9 kPa

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Pressure Measurements

This woman weighs 120 lbs (525 N) and her shoes cover an area (her footprint) of 35 in2 (0.0223 m2). What is the pressure under her feet (in psi and kPa)?

P = F A

= 525 N 0.0223 m2

= 23542 Pa = 23.5 kPa

P = F A

= 120 lbs 35in2

= 3.43 lb in2

= 3.43 psi

handout

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Pressure Measurements

What would happen to the pressure if she changed into high heels?The area covered by her shoes (her footprint) would go down to 20 in2 (0.013 m2).

P = F A

= 525 N 0.013 m2

= 40385 Pa = 40.4 kPa

P = F A

= 120 lbs 20 in2

= 6.0 lb in2

= 6.0 psi

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The Big Picture of Gases

■ The gases of the atmosphere of the Earth exerts a pressure on the Earth and everything on it.

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Earth’s Atmosphere

■ The Earth’s atmosphere is a sea of gas and we are at the bottom.

■ The pressure exerted by the gases of the atmosphere surrounds us.

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Composition of the Atmosphere

■ The gases of the atmosphere are mainly nitrogen (78%) and oxygen (21%) with trace amounts of other gases.

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Weight of Gases Produces Atmospheric Pressure

■ The force of gravity holds the gases of the atmosphere around the Earth and gives the gases weight.

■ The weight of the gases produces a pressure at the surface of the Earth equal to the weight of a 1.03 kg mass per cm2 (10.1 N/cm2).

■ Because of the decreasing weight of the gases above, pressure and density decrease with altitude.

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Atmospheric Pressure

■ The force produced by the weight of the gases of the atmosphere is called atmospheric pressure.

■ Atmospheric pressure at sea level is 101.3 kilopascals (KPa)

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Barometer

■ Atmospheric pressure is measured with a barometer.

■ The first barometer was developed by Italian scientist Evangelista Torricelli (a student of and secretary to Galileo).

■ In a barometer, the pressure of the atmosphere pushes a column of fluid (mercury) up in an evacuated tube.

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Mercury Barometer

■ Changes in atmospheric pressure change the height of the column of mercury in the barometer’s tube.

■ Therefore, atmospheric pressure can also be measured in inches or millimeters of mercury in the tube.

Water could be used in a barometer but since it is 14 times less dense than mercury, the column would be over 34 feet high.

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Units of Measure – Atmospheric Pressure

■ The various means of measurement have led to many different units for measuring atmospheric pressure.

■ Atmospheric pressure at sea level equals:

1.000 atmosphere (atm)

101300 Pascals (Pa)

101.3 kilopascals (kPa)

760.0 millimeters of mercury (mmHg) (760.0 torr)

29.92 in Hg All these measures are equivalent

14.70 pounds per square inch (psi)Memorize these!

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Atmospheric Pressure

■ An atmospheric pressure of 14.70 psi means every square inch of your body has 14.70 lbs pressing down on it.

■ This is equal to over 2000 lbs per square foot!

■ Why don’t we feel it?

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Atmospheric Pressure

■ The pressure of the fluids inside your body equals the atmospheric pressure so you don’t feel the pressure of the atmosphere.

■ Your body is designed to withstand the pressure of the atmosphere.

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Pressure Effects on the Body

■ Pressures much greater or less than atmospheric pressure can harm your body. ■ At great depths, water

pressure forces nitrogen to dissolve in the bloodstream

■ When surfacing, the pressure is reduced and the nitrogen “bubbles” out of your blood

■ This produces a painful condition called “the bends”

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Pressure Effects on the Body

■ If you were in a spacecraft that suddenly lost air pressure, the gases dissolved in your blood would vaporize casing your blood to literally boil.

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Fluid Flow in a Straw

■ What makes a liquid go up a straw?

■ When you suck on the air out of a straw the pressure in the straw is lowered.

■ The pressure of the atmosphere is then higher than the pressure in the straw.

■ So the atmosphere pushes the liquid from a higher pressure area to a lower pressure area – your mouth.

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Converting Between Units of Measure Atmospheric Pressure

■ In chemistry measurements, it is sometimes necessary to take into account atmospheric pressure. So we have to be able to convert from one unit to another

The atmospheric pressure is 30.12 in Hg. What is the pressure in kPa?

30.12 in Hg29.92 in Hg

101.3 kPa = 101.98 kPa

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Converting Between Units of Measure Atmospheric Pressure

The atmospheric pressure is 100.2 kPa. What is the pressure in atm?

= 0.99 atm100.2 kPa101.3 kPa

1 atm

The pressure in a cylinder of gas is 32 psi. What is the pressure in kPa?

32 psi14.7 psi

101.3 kPa = 220.52 kPa

handout

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Pressure Calculations

For proper operation, the pressure in a neon discharge tube (neon light) must be less than 8 torr. The vacuum pump in the lab is rated to achieve a pressure of 0.5 kPa. Can we use this pump to evacuate our tube?

= 3.75 torr0.5 kPa101.3 kPa

760 torr

Since the pump will achieve a pressure of 3.75 torr, it will be suitable for use.

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How Gases Behave

■ Now that we understand the big picture of atmospheric gases and the measurement of pressure, let’s use the kinetic theory to look at how gases behave in response to changes in temperature and pressure.